The Pentagon tends to cite small arms fire, insurgent attacks, and IEDs as the causes of death when U.S. military personnel are killed in Afghanistan. But, once again, the specter of so-called “green-on-blue” – Afghans killed U.S. military personnel – is on the rise. It has been a tough week for U.S. troops in Afghanistan trying to train Afghan forces who occasionally kill them:

— On Tuesday, a pair of gunmen in Afghan army uniforms killed a U.S. soldier and wounded two others in eastern Paktia province.

— On Wednesday, three U.S. troops, a pair from the Army and one from the Air Force, were killed in Kunar province “when they encountered an insurgent who detonated a suicide vest.” While apparently not a classic green-on-blue attack, combat by suicide vest is rare, and highlights the unconventional tactics the insurgents are using.

— Friday morning comes a report that three U.S. troops were killed in “an attack by an individual wearing an Afghan uniform in southwest Afghanistan today,” reportedly in Helmand province.

That terse description hardly does justice to what happened to the trio: according to an Afghan official, the three U.S. special-forces soldiers were invited to dinner with an Afghan police commander at his military checkpoint, where he killed them before fleeing and defecting to the insurgency.

Thirty coalition troops have died at the hands of their Afghan partners, or insurgents clad in Afghan uniforms, so far this year, according to the Associated Press. That’s up from 20 in 2011. Nothing corrodes morale like such attacks, soldiers will tell you.

As an unnamed U.S. military commander in eastern Afghanistan is quoted as saying in a recently-released Army report:

I mean, if you say, it’s so safe in this area but you never go there, then what makes a place safe? Then in another area where there’s a lot of contact, we go there all the time. So maybe we’re providing the insurgents with a target opportunity. So if I stopped going to an area, the sig acts [significant activities] would stop. Does that mean that the area’s safe and that we’ve achieved success?